In prior art, specification U.S. Pat. No. 5,802,810 discloses a wrapping machine for wrapping a plastic foil web around an object to be packaged. The object to be wrapped is usually a load placed on a pallet, which typically is an assembly of the form of a rectangular parallelepiped.
U.S. Pat. No. 5,802,810 mainly describes a crank arm wrapping machine but, according to the specification, the special gripping and cutting device presented in it can also be applied in a ring-type wrapping machine. A ring-type wrapping machine usually has a machine frame supported on a fixed base and comprising a lifting frame which can be moved by a power means along upright vertical columns. A circular guide track is mounted on the lifting frame so as to be vertically movable with it. A film dispensing unit, on which a film web roll can be rotatably mounted, has been arranged to circulate along the circular guide track along a ring-like path around the object to be packaged to deliver plastic film web from the film web roll so as to form a wrapping around the object to be packaged.
U.S. Pat. No. 5,802,810 discloses a gripping and cutting device implemented with two swing arms, which is designed to replace an earlier gripping and hot seaming device, which at the end of the wrapping operation melts the film web onto the film web layer below by means of an electrically heated resistance wire and then cuts the film web beside the seaming line.
The function of the gripping and cutting device described in specification U.S. Pat. No. 5,802,810 is to hold the end of the film web at the start of the wrapping operation until it is caught between at least one film web layer and the object so that it will stay there. At the final stage of the wrapping operation, the gripping and cutting device is to terminate or cut the film web and leave the said end between at least one film web layer wrapped and the object. After that, the free end of the severed film web remains in the grip of the gripping and cutting device to be ready for the start of the next wrapping operation. This type of gripping and cutting device comprises a device frame. Further, the gripping and cutting device comprises a first swing arm, which is pivotally connected to the device frame via a first joint allowing a turning motion between a horizontal position and a vertical position. The first swing arm also comprises a first holding device for gripping and holding the film web. A second swing arm is pivotally connected to the device frame via a second joint disposed at a distance from the first joint and allowing a turning motion between a horizontal position and an upright position. The second swing arm comprises a second holding device for gripping and holding the constricted film web and a cutting device for culling the film web. This prior-art gripping and cutting device is mounted on the device frame, which is supported on a fixed base, e.g. a floor, and is disposed in a wrapping station, where it can be moved to a position near the lower part of the object to be wrapped.
In itself, the above-mentioned swing-arm type gripping and cutting device has many advantages as compared with the prior-art gripping and seaming device that uses a melting technique. It comprises fewer moving parts and less control. In the device based on a melting technique, the resistance wire has to be supplied with an electric current to heat it, and on the other hand this heating has to be controlled. In the swing-arm type device, only the turning motion of the arms has to be controlled to make them turn at the right time. In addition, in the use of the swing-arm type device, the tension of the film web may vary, whereas for the operation of the seaming device using a melting technique it is necessary that that film web should be under a certain tension, i.e. not too tight and not too loose. It is therefore advantageous to use the swing-arm type gripping and cutting device because it is simpler in structure, requires less control and is not sensitive to variations in film tension.
However, the wrapping machine according to specification U.S. Pat. No. 5,802,810 involves the problem that, as the aforesaid gripping and cutting device implemented using swing arms is disposed near the lower part of the object to be wrapped, the wrapping action always has to be started from the lower part and finally ended at the lower part. Thus, wrapping is started from below, the film web is wrapped around the object in an ascending spiral form. After the object has been wrapped up to its top, more film web is then wrapped around the object in a descending spiral form to reach the lower part of the object again so that the film web can be terminated. Thus, when the prior-art device is used, the object always has to be wrapped twice over. Usually a single wrapping would be sufficient, so with the prior-art device an unnecessarily large amount of film web per object to be wrapped is consumed. In addition, double wrapping requires a long time. A further problem is that, with the prior-art wrapping machine, a so-called surface wrapping can only be made at the lower part of the object to be wrapped. Surface wrapping means that the film is not wrapped in spiral form but on a single given surface to form a ring-like collar around the object.